Tanks and containers for motor vehicle fuels currently are produced by one of two competing technologies. One comprises forming two or more pieces of coated steel into mating "top" and "bottom" portions and then combining the portions with classical metal joining technologies. A second comprises blow molding of a thermoplastic polymer melt. The thermoplastic melt is generally based on a polyolefin such as high density polyethylene for cost, processing and compatibility reasons. Plastic tanks, currently considered more expensive, have significant design and flexibility advantages compared with metal tanks.
There exists an ever-increasing concern with automotive generated pollutants. One aspect of the problem is reduction in simple evaporative emissions of raw fuel from the storage tank and delivery system. Simple monolayer plastic tanks suffer from fuel permeation and excessive evaporative emissions. However, to capture the design advantages of plastics, expensive barrier technologies such as fluorination, sulfonation or coextruded barrier layers are used to control evaporation. Metal tanks, while impervious to evaporation through the major surface portions, are susceptible to evaporative escape through joints and seams, especially as these features deteriorate with age.
The California Air Resource Board (CARB) has issued new standards regarding automotive emissions to take effect in 1996. The new standards regarding evaporative emissions from vehicle fuel systems are approximately ten times more stringent than the current (1993) evaporative standards. The plastic barrier technologies as classically practiced appear only marginally acceptable in meeting the new evaporative standards.
The new CARB standards also focus on a reduction in polluting combustion gases and therefore will likely require cleaner burning fuels. Cleaner burning fuels can be formulated by blending 10% to 20% of an oxygenated hydrocarbon such as methanol or ethanol into the fuel mix. However, the anticipated introduction of oxygenated fuels presents unique problems regarding evaporative escape. Methanol, and to a greater extent ethanol, increase volatility of the raw fuel, resulting in increased evaporative escape. The volatility impact of oxygenated additives makes it increasingly difficult, if not impossible, to achieve the rigid new CARB evaporative standards with plastic tanks using classical barrier technologies. Metal tanks also suffer in the presence of oxygenated fuel additives from all increase in interior corrosion attack, promoting premature deterioration. These various problems are thoroughly reviewed in Plastics Technology Magazine, May, 1992, pages 52-57.
The entire issue of oxygenated fuels is further complicated by economic and political forces. Methanol is produced from petroleum. Since methanol has less effect on volatility than ethanol, consideration of emissions alone would lead to a choice of methanol as the primary oxygenated additive. However, ethanol is an agricultural product whose use would be a huge economic benefit to farmers and preserve petroleum stocks. Those knowledgeable in the art are quick to point out that the increased evaporation resulting from ethanol containing fuels could well overcome the positive effects of decreasing harmful combustion products. Despite this concern, the political and economic benefits of ethanol use as an oxygenated additive have resulted in a U.S. Government decision to permit ethanol use in automotive fuel blends. This issue has been widely discussed in the U.S. press, for example U.S.A. Today, Oct. 2, 1992.
There is an immediate need for an improved gasoline tank technology that would be capable of allowing the use of oxygenated fuel additives without deleterious evaporative effects while maintaining the cost and design advantages associated with existing tank technologies.
In seemingly unrelated technology, electroplating on plastic substrates has been employed to achieve decorative effects on items such as knobs, cosmetic closures, faucets and automotive trim. ABS (acrylonitrile-butadiene-styrene) plastic dominates as the substrate of choice for most applications because of a blend of mechanical and process properties and ability to be uniformly etched. The overall plating process comprises many steps. First, the plastic substrate is chemically etched to microscopically roughen the surface. This is followed by depositing an initial metal layer by chemical reduction. This initial metal layer is normally copper or nickel of thickness typically one-half micrometer. The object is then electroplated with metals such as bright nickel and chromium to achieve the desired thickness and decorative effect. The process is very sensitive to fabrication processing of the plastic substrate, limiting applications to carefully molded parts and designs. In addition, the many steps employing harsh chemicals make the process intrinsically costly and environmentally difficult. Finally, the sensitivity of ABS plastic to liquid hydrocarbons has prevented certain applications.
Certain printed circuits are produced using the chemical metal reduction techniques which comprise the initial portion of the overall decorative plating of ABS plastic. In the case of printed circuits, the metal layer can be relatively thin because it is protected by enclosures and need not exhibit decorative effects.
The conventional technology for electroplating on plastic (etching, chemical reduction, electroplating) has been extensively documented and discussed in the public and commercial literature. See, for example, Saubestre, Transactions of the Institute of Metal Finishing, 1969, Vol. 47., or Arcilesi et. al., Products Finishing, March, 1984.
Many attempts have been made to simplify the process of electroplating on plastic substrates. Some involve special chemical techniques to produce an electrically conductive film on the surface. Typical examples of this approach are taught by U.S. Pat. No. 3,523,875 to Minklei, U.S. Pat. No. 3,682,786 to Brown et al., and U.S. Pat. No. 3,619,382 to Lupinski. The electrically conductive film produced was then electroplated.
Another approach to simplify electroplating of plastic substrates is incorporation of electrically conductive fillers into the resin to produce an electrically conductive plastic. The electrically conductive resin is then electroplated. Examples of this approach are the teachings of Adelman in U.S. Pat. No. 4,038,042 and Luch in U.S. Pat. No. 3,865,699. The Adelman approach included pre-etching to achieve adhesion of the electrodeposit. Luch taught incorporation of small amounts of sulfur into the polymer compound to produce a chemical bond between the plastic substrate and electrodeposit. None of the above alternate approaches to conventional electroplating of plastics has achieved widespread commercial application.
Numerous other attempts have been made to impart certain metallic properties to plastics, especially electrical conductivity. The impetus for many of these efforts is the increasing emphasis on shielding of electromagnetic radiation. These other attempts include metal-filled conductive paints, metal sprays, foils, metallized sheets, silver reduction, vacuum metallizing and cathode sputtering.